1. Field
The present invention relates to gas permeable membranes and, more particularly, relates to gas permeable polymeric membranes or composite membranes comprising at least one glassy polymer, which membranes are preformed and subjected to conditions wherein a chemical reaction such as an oxidative reaction takes place throughout the membrane. The resulting membrane manifests enhanced selectivity for at least one pair of gases, vapors, or molecules (permeating by solution diffusion through the glassy polymeric membrane) as compared to the intrinsic selectivity of the polymer, or polymers, making up the membrane, and as compared to the equilibrium intrinsic selectivity of the chemically modified polymer or polymers, for the same selected pair of gases, vapors or molecules. In a preferred embodiment, an asymmetric hollow fiber membrane comprising a glassy polymer, such as polysulfone, is ozone-treated in order to enhance the selectivity thereof for a pair of gases, as compared to the intrinsic selectivity of the polysulfone, and as compared to the equilibrium intrinsic selectivity of the ozone-treated polysulfone for the same pair of gases. 2. Related Art
Several methods have been developed for enhancing the selectivity of fluid permeable membranes by changing the surface characteristics thereof. For example, Janssen et al., U.S. Pat. No. 4,968,532, disclose ozone treatment of a preformed polymer substrate which is saturated or swollen with a liquid in order to graft polymerize a monomer to the surface of said substrate and thus modify only the surface characteristics thereof. Alternatively, the substrate is treated with ozone and then saturated or swollen with a liquid prior to exposure to a graft monomer. See also U.S. Pat. Nos. 4,311,573 and 4,589,964.
Shimomura et al., U.K. 2 089 285, disclose gas separation membranes obtained by exposing a porous hollow fiber to a plasma consisting of a gaseous organic compound, an inorganic gas or a mixture thereof in order to form a dense cross-linked layer on the surface of such membrane.
Brooks et al., U.S. Pat. No. 4,575,385, disclose membranes having improved permeation selectivities wherein an asymmetric gas separation membrane is contacted on one or both surfaces with an effective amount of an aromatic permeation modifier. Murphy, U.S. Pat. No. 4,728,346, discloses coated membranes having improved permeation selectivities wherein an asymmetric gas separation membrane is contacted on one or both surfaces with an aromatic permeation modifier and combined with a coating. See also U.S. Pat. No. 4,654,055, to Malon et al, wherein a membrane is contacted on one or both sides with a Bronsted-Lowry base which does not produce chemical changes in the polymer, and U.S. Pat. No. 4,486,202 wherein a membrane is treated with a Lewis acid.
Selectivity enhancement of membranes through modifications to the surface thereof are limited in that only a limited percentage of the surface can be modified without affecting the overall physical characteristics of the membrane and further in that the modification is specifically limited to conditions under which the chemical bonds are stable. It has now been discovered that selectivity enhancement can be achieved, and to a much greater extent, by modifying the interstices or recesses of the membrane substantially throughout the thickness thereof.
While other methods (especially those of surface modification) have been shown to improve the selectivity of specific membranes, such methods are generally accompanied by significant decreases in the permeability of the faster specie. The current method provides significant selectivity increases for many polymers with very minimal loss in permeability for the modified polymer.